Experiments will be done in vitro to determine the role that cells play in the induction and maintenance of unresponsiveness to skin allografts in ALS-treated mice injected with bone marrow. Spleen cells from ALS-treated B6AFl mice grafted with C3H/He skin and injected with C3H/He marrow will be compared to spleen cells from ALS-treated grafted controls in their ability to inhibit the proliferative response in mixed lymphocyte cultures and the generation of cytotoxic reactivity. If suppressor cells are found, attempts will be made to define the cells population responsible for suppression, that is T or B cell macrophage. In addition, the suppressor cells will be assayed for specificity to see if they are specific for C3H/He antigens or non-specific in their ability to suppress proliferation in mixed lymphocyte cultures. Experiments will also be designed to determine whether the suppressor cells originate from the host or from the donor marrow. Experiments will be continued on the effect of blood transfusions on subsequent skin allograft survival. Whole blood will be separated into its components parts, i.e. red cells, white cells, platelets and plasma, to determine the ccmponents responsible for the enhancing effect. In addition, various combinations of donor and recipient strains will be used to investigate the importance of histocompatibility differences between blood and skin donors and recipients.